The invention concerns a test strip for the optical or electrical current determination of the concentration of a substance in a liquid, especially for blood sugar determination, including a strip-shaped carrier with a reaction field which contains a reagent, and which upon wetting with the substance to be investigated changes the optical transmissivity or reflectivity or electrical conductivity of the reaction field.
For blood sugar determination, such as a diabetic under certain circumstances must take several times a day, the patient has to stick himself in the finger to obtain blood, a drop of which is applied to the test strip. This procedure is not only unpleasant for the patient, but in the course of time leads to the finger becoming so scarred that the obtaining of a sufficiently large drop of blood becomes difficult. Therefore, it becomes desirable to have test strips making possible a sufficiently precise measurement from a very small amount of blood. In connection with this, it is to be taken into account that the test strips, because of being hand manipulated in a measuring procedure, cannot be made as small as might otherwise be desired. Also, to be taken into account is that the test strips should be constructed in such a way that they can be made by means of a continuous process.
The invention, therefore, has as its object the provision of a test strip of the above-mentioned kind which on one hand delivers a trouble free measuring result with a small volume of the investigated liquid and which on the other hand can be made in a rational way.
For the solving of the above object, it is proposed in accordance with the invention that the test strip carrier be made of an optically transparent material onto which a reagent containing thin layered reaction carrier is applied. This reaction carrier is preferably a paint, but can also be a paste or a thin paper.
In customary test strips, the reagent is contained in a so-called membrane, consisting of a felt, foam material, or textile piece, which absorbs the fluid to be investigated. Therefore, a substantial amount of liquid is needed to soak the entire membrane. In contrast to this, in the inventive solution, the thin layered reaction carrier itself takes on only very little liquid. The fluid dropped onto the reaction field reacts directly with the reagent. Thereby, in comparison to the usual test strip only a very small amount of liquid is needed for the desired concentration determination. Further, the making of the strips is of utmost simplicity, since only one web is needed with the carrier material, onto which web the reagent containing reaction carrier is applied as paint or paste, and from which web the test strips subsequently can be cut.
Whereas, in customary test strips, the size of the measuring field is determined by the dimensions of the membrane piece, in the case of the inventive solution the reaction field is not sharply limited. To obtain a clearly limited reaction field and measuring field for the optical measurement, it is advantageous if the reaction field is bordered by a diaphragm, for example, of two opaque diaphragm strips running along the longitudinal edges of the strip or of an opaque surrounding diaphragm. This diaphragm or the diaphragm strips can simply be printed onto the carrier web, for example, with black color. Advantageously, the diaphragm strips are first printed onto the carrier web, and then after that the reagent containing reaction carrier is applied. The above described advantage of the transparent carrier web provided with a diaphragm is obtained also if the reaction carrier is formed by a customary membrane.
The transparent carrier web offers the possibility of providing on the carrier web itself a reference surface which advantageously is arranged in the surface area of at least one of the diaphragm strips. Therefore, with the help of a corresponding optic system, upon the depositing or insertion of the test strip into the measuring device, a calibrating measurement can be carried out at the reference surface. This has not only the advantage that the reference measurement and the actual concentration measurement can be performed at practically the same time, but also that the spacing of the reference surface from the measuring optic system is identical to the spacing of the reaction field from the measuring optic system. If, for example, the test strip does not lie entirely flatly on the strip support surface, this error occurs in the same way for the reference surface as well as for the reaction field, so that the error is compensated for by the comparing measurements.
Preferably, the reference surface is applied directly onto the carrier web material and indeed before the printing of the diaphragm strips.
Advantageously, the diaphragm strips and/or the reference surface are provided on the same side of the carrier web as is the reagent containing paint. The diaphragm can, however, also be printed onto the opposite carrier web side.
To achieve a uniform distribution of even the smallest liquid drop over the reaction field, the reaction field can be covered by a hydrophilic material. For this can be used, for example, a finely woven textile or a fine netting having practically no thickness so as to xe2x80x9csuck upxe2x80x9d very little liquid in comparison to the customary membranes. To, on one hand, limit the liquid absorption by the hydrophilic material and, on the other hand, to not make difficult the continuous manufacture of the test strips, the hydrophilic material can be made or be treated so as to have hydrophobic surface portions in which surface portions no liquid can be absorbed. At the same time, there results a spatial limiting of the blood application location over the measuring optic system.
To limit the blood application area and to protect the reaction field, the surface area of the test strip containing the reaction field can be provided with a hydrophobic cover having a drop application opening. In this case, the drop application opening does not have to lie directly over the measuring field onto which the measuring optic system is directed. In a preferred embodiment of the invention, the drop application opening is on the contrary provided adjacent to a longitudinal end of the test strip. This provides the possibility of inserting the test strip into the measuring device and not until then applying the fluid to be investigated, for example, by stripping off a blood drop in the region of the drop application opening at the end of the strip. Because of the hydrophilic material, the fluid to be investigated is then transported from the drop application opening to the area of the reaction field detected by the measuring optic system. The cover is preferably a plastic foil.
The invention further concerns a measuring device for the optical determination of the concentration of a substance in a liquid, especially for blood sugar determination, by means of a test strip of the previously described kind, wherein the measuring device has a housing with a strip support surface, a measuring optic system or contacts arranged below the strip support surface, an indicator mechanism, an operating field and an evaluation and control circuit. According to the invention, the measuring optic system includes three optical sensors for the optical measurement, of which sensors two are directed onto the reaction field and one is directed onto the reference surface. This makes possible the simultaneous measurement of the reference surface and of the reaction field, so that not only is the carrying out of the measurement simplified, but also the exclusion of error sources is possible.
In a preferred embodiment, the sensors directed toward the reaction field are arranged behind one another in a longitudinal direction of a test strip lying on the strip support surface. If, for example, the liquid is applied in the above-described way to the drop application opening provided at the longitudinal end of the strip and is transported by the hydrophilic material in the direction toward the reaction field, a measuring signal from the two spaced sensors further give the assurance that the region of the reaction field lying between the second sensor and the drop application opening is entirely wetted with the fluid to be investigated and that the output signal of the first sensor lying between the second sensor and the drop application opening corresponds to a fully wetted surface.
In a similar way in case of a test strip to be measured by way of electrical current, a test can be made as to whether the test field has been sufficiently wetted with the liquid to be investigated, if the electrodes for an electrical current measurement of the measuring field of the test strip lie between the drop application location and an optical measuring point. If at the optical measuring point, it is determined by the help of an associated optical sensor that the liquid to be investigated has reached the optical measuring point, it can be taken from that that a sufficient amount of the liquid to be investigated has also reached the measuring field in which the measuring electrodes lie.
To be able to compensate for a change in the sensitivity of the optical sensors with or in dependence on temperature, in accordance with the invention it is proposed that a reference surface is provided above the strip support surface for balancing of the optical sensors. In this case, the evaluation and control circuit can be so made that it carries out the comparison at pre-given time intervals or that the comparison process is carried out in dependence on a pre-given temperature change, which change is determined by the help of a temperature sensor provided in the measuring device.
The reference surface can be formed directly onto a cover fixed to the housing and extending over the strip support surface. Another possibility is that the reference surface can be formed on a key which is so arranged on the housing above the strip support surface so that it is movable between a position at which it presses the test strip against the support surface and a test strip freeing position. This key can be at least in part transparent.
Preferably the evaluation and control circuit is so formed that by means of the optical sensors different reflectivities/transmissivities of the field are recognizable. This offers the possibility of calibrating the measuring optic system of the measuring device so that an actual measured reflectivity/transmissivity of the measuring field also corresponds actually to a given concentration of the investigated substance. The functional correlation between the reflectivity/transmissivity detected by the measuring optic system and the concentration values of the investigated substance is stored in the evaluation and control circuit.
The calibration can be done in that a card can be arranged on the strip support surface and have calibration fields associated with the individual optical sensors, the calibration fields having reflectivity/transmissivity values each corresponding to a given concentration of the investigated substance, so that the actual measured reflectivity/transmissivity values of the calibration field can be compared with the corresponding values according to the stored functional relationship and so that upon the appearance of a difference a corresponding correction factor can be determined by means of which a test strip measured reflectivity/transmissivity value can be corrected.
The calibration card can moreover carry a code evaluatable by the evaluation and control circuit, which code, for example, contains information about the selected measuring program or the batch from which the test strip to be measured came. This code can be formed with the help of the calibration fields. The calibration card can also carry other information. Especially, further color fields can be provided which make possible a visual comparison between the measuring field of a test strip and one of the color fields, so at least a coarse determination of the concentration can be carried out visually.